1. Field of the Invention
This invention relates to a composite coating steel sheet and particularly to a plated base steel sheet provided thereon with multiple coatings to improve corrosion resistance, paintability and corrosion resistance after paint coating.
2. Description of the Prior Art
Chromate treatment has been commonly employed as rust preventive treatment of galvanized steel sheets, zinc alloy plated steel sheets and aluminum plated steel sheets. In most cases, such treatment is intended merely for a temporary rust preventive measure and the corrosion reistance thereby obtained is at a low level such that white rust appears in 24 to 48 hours when the treated steel sheets are subjected to a salt spray test. Accordingly, in a case where the products are intended for use under a severe corrosive environment for a long period of time, there has been no practical way other than applying a thick paint coating at a level of more than 10.mu. to prevent corrosion. However, in recent years the costs of paints have been rapidly raised reflecting the increase in price of petroleum products, and it is strongly desired to develop steel sheet products having good corrosion resistance, which can be produced simply by surface treatments without using paints.
Under the circumstances, there has been a certain development in the art of chromate treatment. A typical one is a coating type chromate treatment in which various binders are added to the chromate treatment solution, whereby products having improved corrosion resistance are actually produced. Even in this case, however, the corrosion resistance thereby obtainable is at best at a level of 200 hours against formation of white rust when subjected to a salt spray test.
Even when paint coating is required to provide an aesthetic outer appearance, it is usual to attempt to cut down the costs for the paint by choosing a low grade paint or by minimizing the thickness of the paint coating. In such a case, the steel sheets are required to have not only high corrosion resistance but also good paintability and corrosion resistance after paint coating. It is necessary that these properties are well balanced.
However, there has been no chromate treated products which fully satisfy these requirements. Some products which satisfy the requirement for high corrosion resistance, tend to fail in providing adequate paintability or corrosion resistance after paint coating. Whereas those having good paintability tend to fail in the requirement for high corrosion resistance.
For example, so-called unichrome treatment is known whereby it is intended to provide high corrosion resistance without paint coating by means of a reactive chromate. According to this method, a plated steel sheet is dipped in a treatment solution composed of chromic acid and a mineral acid for a long period of time to form a chromate film coating having a thickness of from about 500 to about 700 mg/m.sup.2 as calculated as the amount of metal chromium. However, such a treatment has drawbacks that the dipping process requires a long period of time and the degradation or ageing of the treatment solution is rapid, and it is totally unsatisfactory as a strip coating technique. Further, as the chromate film is rather thick, it is susceptible to cracking and its paintability is not good. If the thickness of the chromate film is decreased to a level of 100 mg/m.sup.2, the film cracking may be avoided, but the corrosion resistance is reduced to a level of 100 hours, thus loosing the high corrosion resistant characteristics.
On the other hand, in coating type chromate treatment, a binder is used to fix or trap a substantial amount of chromium therein and to prevent the film cracking by the binder. However, it has a drawback that the treatment solution is susceptible to gelation, and even when it is not susceptible to gelation, it tends to form a thick film which requires a special treatment for drying.
In recent years, it has been proposed, for the purpose of avoiding the above mentioned difficulties, to form a double layer coating by duplex plating treatment. Namely, a thin chromate film is formed as the first layer in a short period of time, and then an inorganic or organic substance is coated thereon to form a film as the second layer which protects the first layer of the chromate film. Use of inorganic materials is disclosed in Japanese Laid-Open Patent Application No. 9545/75 and Japanese Patent Publication No. 19981/78 in which a chromate film is electrolytically formed as the first layer and a treatment solution comprising a chromic acid and silica sol is applied and then dried to form a film as the second layer. High corrosion resistance is obtainable by forming the second layer sufficiently thick. However, the film thereby obtained is apt to be peeled off because of the thick silica layer and thus lacks in durability after processing. Further, as the silica sol has poor affinity to a paint, the treated surface is not satisfactory as a substrate for paint coating.
A typical example in which an organic material is used for the second layer, is Japanese Patent Publication No. 35620/77, in which a chromimum hydrated oxide layer is formed as the first layer and then a water soluble organic resin layer is formed as the second layer. However, adequate effectiveness cannot be expected from the organic resin layer disclosed therein, since the functional groups present in the organic resin of the second layer tend to attract water and therefore it is impossible to obtain a high level of corrosion resistance. Even if the thickness of the second layer is increased as much as 1.mu., a high level of corrosion resistance will not be obtainable, and in such a case, it will be difficult to completely dry the film by hot air only and a special apparatus for drying will be required.
Further, in Japanese Patent Publications No. 36100/74, No. 18445/75, No. 4611/74 and No. 1986/74, it is proposed to form a chromate film as the first layer and an organic film as the second layer. However, in each of these cases, the functional groups in the organic resins in the second layer tend to attract water and it is impossible to obtain a high level of corrosion resistance. These products are presumably effective rather as a substrate for paint coating taking the advantage of the functional groups.
As mentioned above, the products having a second layer formed with an inorganic or organic material are effective either for the corrosion resistance without paint coating or for the paintability, but they can not satisfy both of the required properties at the same time. It is conceivable to form the second layer with use of a mixture of an organic material and an inorganic material. However, a mere mixture does not provide better results, and to the contrary, it is likely in many cases that desirable properties of the individual components are imparied.
Thus, it has been difficult to produce a chromate treated steel sheets which are substrates for paint coating and which at the same time have superior corrosion resistance.